Mearns was born to Alexander and Nancy Reliance Mearns née Clarswell. His grandfather Alexander was of Scottish origin and moved to Highland Falls in 1815. Edgar Alexander Mearns was educated in the Donald Highland Institute (Highland Falls) and attended the Columbia College of Physicians and Surgeons where he graduated in 1881.

In 1881, he married Ella Wittich of Circleville, Ohio. The couple had one son and one daughter. Their son was born in 1886 and died in 1912. From 1882 to 1899 he served the military service as a surgeon. From 1899 to 1903, he was a medical officer in several army institutions. From 1903 to 1904 and 1905 to 1907, he traveled to the Philippines. He had to interrupt his journey in 1904 because he came down with a parasitic disease. In 1905 a trip led him to Guam.[1]

As Major and Surgeon in the Army, Mearns was appointed medical officer to the International Boundary Commission and reported on the fauna and trees of the boundary between Mexico and the United States in his 1907, Mammals of the Mexican Boundary of the United States.[2] In 1909 he retired from the army with a rank of a Lieutenant Colonel, and later that same year he was invited by Theodore Roosevelt to accompany the Smithsonian-Roosevelt African Expedition as naturalist.[3] From 1911 to 1912 he was a member of the Childs Frick expedition in Africa to collect and prepare specimens of birds that Frick later presented to the Smithsonian Institution.[4]